1. Field of the Invention
The present invention relates to an antenna device including a metal casing as an antenna and an electronic apparatus including the same.
2. Description of the Related Art
In recent years, a casing made of a metal such as aluminum or magnesium has sometimes been used in a mobile communication terminal such as a cellular phone terminal to improve the exterior design and maintain the strength of the casing, which is becoming larger and thinner.
If an electronic apparatus including an antenna has a metal casing, electromagnetic waves transmitted or received by the antenna are subject to the influence of, for example, reflection and absorption by the metal casing. When mounting the antenna in or on the electronic apparatus including the metal casing, therefore, the configuration, disposition, and so forth of the antenna need to be elaborated. For example, in an HF-band communication system such as NFC (Near Field Communication), simply disposing a coil antenna inside a metal casing does not enable communication with a communication partner.
Meanwhile, antenna devices using a metal casing as an antenna include the one described in International Publication No. 2010/122685. The antenna described in International Publication No. 2010/122685 includes a metal casing portion and a feed coil. A main surface of the metal casing portion is formed with an opening portion and a slit portion. A coil opening portion of the feed coil and the opening portion of the metal casing portion overlap, as viewed in the direction perpendicular to the main surface.
With this configuration, when a current flows through the feed coil, an induced current flows around the metal casing portion. Further, a magnetic field passes through the opening portion and circulates to surround the metal casing portion. Therefore, the magnetic field widely spreads around the metal casing portion, allowing the metal casing portion to function as a part of the antenna.
In the antenna device described in International Publication No. 2010/122685, it is necessary to form the opening portion and the slit portion in the main surface of the metal casing portion. However, the shape and so forth and the main surface of the electronic apparatus have significant influence on the design of the electronic apparatus. The above-described antenna device, therefore, may limit the degree of design freedom of the electronic apparatus including the antenna device and compromise the design. Further, if the opening portion and the slit portion are formed in the main surface of the metal casing portion, the strength of the casing is substantially reduced. According to the above-described antenna device, therefore, a casing having sufficient strength may not be obtained.